A standard dual-frequency identiﬁcation sonar (DIDSON) was deployed in the Herring River, Harwich, Massachusetts, for 3 d in late April 2011 to capture video-like images of migrating adult river herring (alewifeAlosa pseudoharengusand blueback herringAlosa aestivalis). Images recorded 24 h a day were used to manually count and assign species based on DIDSON images of ﬁsh size, shape, and behavior. From these counts, the run size was estimated to be 1,976–2,059 individuals during the study. At ﬁrst, river herring often hesitated to swim through the sample area where the weirs and DIDSON were deployed; however, they eventually did pass, often multiple times. This unique hesitation behavior complicated counting efforts, though it was beneﬁcial to discerning species using DIDSON images. In addition, extremely shallow water upstream of the study site, lack of tree cover, and a high threat of avian predation likely contributed to river herring milling activity at the study site. By providing many clear images of river herring, DIDSON proved to be an effective type of sonar with which to monitor and count river herring continuously in a small coastal stream.
The primary objective of this project was to assess the ability of dual-frequency identiﬁcation sonar (DIDSON) to monitor migrating anadromous alewife Alosa pseudoharengus and blueback herring Alosa aestivalis(collectively known as “river herring”) in a small Massachusetts coastal stream. Since 2006, a moratorium on river herring harvest has been in place in Massachusetts due to concerns about low population levels (Massachusetts Division of Marine Fisheries, Regulation of Catches, River Herring 2006). Annual river herring population estimates are largely based on data collected by dedicated volunteers, who count upstream-migrating river herring at ﬁsh passage structures each spring as these ﬁsh leave the ocean seeking freshwater spawning grounds. Since it is unrealistic to count ﬁsh every minute of each day, the volunteers often employ some type of random sampling design to set up a sampling schedule (Nelson 2006). Volunteer river herring counts are then extrapolated to the daylight hours (0700–1900 hours) to estimate the daily run size from early April to mid-June (Nelson 2006).
In this project, Barnstable County Cooperative Extension and
Woods Hole Sea Grant collaborated with the Harwich Conservation Trust and the Town of Harwich, Massachusetts, to evaluate the ability of DIDSON to image river herring. DIDSON is a relatively new sonar technology designed by Sound Metrics Corp. in Lake Forest Park, Washington. When used in high-resolution (1.8-MHz) mode, the standard DIDSON unit constructs videolike images using 96 acoustic beams that combine to produce a ﬁeld of view that is 29 degrees wide×14 degrees high (Belcher et al. 2001). Frames are typically recorded at a rate of 5–21 per second, with a series of frames producing a detailed video that captures ﬁsh movements and shape (Belcher et al. 2001). DIDSON is widely used in Paciﬁc coastal streams to enumerate Paciﬁc salmon Oncorhynchus spp. (Pipal et al. 2010). A study by Maxwell and Gove (2007) of the Alaska Department of Fish and Game showed that DIDSON counts of migrating Paciﬁc salmon were similar to visual observations at the same site. Fish smaller than adult Paciﬁc salmon can also be detected by the DIDSON; while observing baited ﬁshing pots, Rose et al. (2005) noted that DIDSON can image and track ﬁsh as small as 20 cm at a sample range of 9 m. Twenty to thirty centimeters (total length) is roughly the size range of adult river herring in springtime migrations in Massachusetts (Nelson et al. 2011).
Subject editor: Donald Noakes, Thompson Rivers University, British Columbia, Canada
*Corresponding author: firstname.lastname@example.org
Received January 19, 2012; accepted July 30, 2012